1. Field of the Invention
The present invention relates to a phosphor, and more particularly to an alkaline earth metal aluminate phosphor activated by europium (Eu). The present invention also relates to a plasma display panel (PDP) that has a phosphor layer using the phosphor.
2. Description of the Related Art
In recent years, alkaline earth metal aluminate phosphors have been applied as low energy phosphors in 3-wavelength fluorescent lamps and the like. The following compositions are specific examples of such phosphors.
Blue Phosphor: (Ba, Sr)MgAl10O17:Eu
Green Phosphor: CeMgAl11O19:Eu or BaMgAl10O17:Eu, Mn
These phosphors are used separately or mixed with different phosphors or a red phosphor so as to achieve an emission color such as white luminescence.
As a blue phosphor for PDPs, (Ba, Sr)MgAl10O17:Eu having high wavelength conversion efficiency in the vacuum ultraviolet region has been utilized. By setting a molar quantity of Eu in a range of 5 mol % to 15 mol % of a total molar quantity of (Ba, Sr) and Eu, it is possible to achieve superior luminescence characteristics.
With the above lamps and PDPs, slurry made by mixing binder with the phosphor is applied to a substrate (e.g. glass) and baked to form a luminous screen consisting of phosphor layers (phosphor films).
It has been noted that the wavelength conversion efficiency of (Ba, Sr)MgAl10O17:Eu deteriorates with time, depending on how the device is used.
Various methods have been proposed in order to suppress the deterioration. For example, Japanese Laid-Open Patent Application No. H6-29418 discloses a method of adding 5 mol % or smaller amount of gadolinium to a phosphor material.
Further, Japanese Laid-Open Patent Application No. 2000-34478 discloses a method of coating a surface of phosphor particles with divalent metal silicate such as alkaline earth metal. In addition, Japanese Laid-Open Patent Application No. H10-330746 discloses a method of coating the surface of the phosphor particles with antimony (Sb) oxide.
Moreover, Japanese Laid-Open Patent Application No. 2003-213258 discloses that the proportion of divalent europium in the europium contained in (Ba, Sr)MgAl10O17:Eu is set in a range of 45% to 85%.
With the methods disclosed in No. H6-29418 and No. 2000-34478, however, the deterioration caused by the vacuum ultraviolet irradiation is not suppressed efficiently, even though the deterioration due to heat may be effectively suppressed.
In addition, with the method disclosed in No. H10-330746, it is not very easy, in practice, to evenly cover the surface of the phosphor particles with an Sb oxide film. It is also difficult to improve chromaticity shift and emission intensity maintaining ratio at the same time, because chromaticity shift and emission intensity maintaining ratio have an inverse relationship.
Moreover, with PDPs in which the above phosphors are used in the phosphor layers, so called “sticking image” is caused. The sticking image is a residual image of a specific color (an image in which blue is not properly displayed) that is continuously displayed, when a PDP is driven, as if the image was burnt in. The sticking image is caused because the chromaticity shift and the emission intensity of the Eu-activated phosphors change with time with relative ease. Such a sticking image impairs display performance.
Furthermore, the inventors of the present invention performed detailed investigations and have found that, in some cases, setting the proportion of the divalent Eu in the total amount of Eu in one phosphor particle as in No. 2003-213258 is not sufficient to achieve the intended effects such as suppressing the deterioration caused by heat during manufacture of the phosphor layers and such, and by vacuum ultraviolet irradiation when displaying images.
As described above, it is considered that further improvements are necessary in order to suppress the deterioration of the phosphors.